This invention relates generally to a way to improve the transfer of water vapor produced by a fuel cell, and more particularly to an improved water vapor transfer (WVT) separator plate assembly and a method for making such an assembly.
In many fuel cell systems, hydrogen or a hydrogen-rich gas is supplied through a flowfield to the anode side of a fuel cell while oxygen (such as in the form of atmospheric oxygen) is supplied through a separate flowfield to the cathode side of the fuel cell. An appropriate catalyst (for example, platinum) is typically disposed as a layer on porous diffusion media that is typically made from a carbon fabric or paper such that it exhibits resiliency, electrical conductivity, and gas permeability. The catalyzed diffusion media is used to facilitate hydrogen oxidation at the anode side and oxygen reduction at the cathode side. An electric current produced by the dissociation of the hydrogen at the anode is passed from the catalyzed portion of the diffusion media and through a separate circuit such that it can be the source of useful work, while the ionized hydrogen passes through the MEA to combine with ionized oxygen at the cathode to form high temperature water vapor as a reaction byproduct. In one form of fuel cell, called the proton exchange membrane or polymer electrolyte membrane (in either event, PEM) fuel cell, an electrolyte in the form of a perfluorinated sulfonic acid (PFSA) ionomer membrane (such as Nafion®) is assembled between the diffusion media of the anode and cathode. This layered structure is commonly referred to as a membrane electrode assembly (MEA), and forms a single fuel cell. Many such single cells can be combined to form a fuel cell stack, increasing the power output thereof.
Fuel cells, particularly PEM fuel cells, require balanced water levels to ensure proper operation. For example, it is important to avoid having too much water in the fuel cell, which can result in the flooding or related blockage of the reactant flowfield channels, thereby hampering cell operation. On the other hand, too little hydration limits the electrical conductivity of the membrane and can lead to premature cell failure. Exacerbating the difficulty in maintaining a balance in water level is that there are numerous conflicting reactions taking place in a fuel cell that are simultaneously increasing and decreasing local and global hydration levels.
One method of ensuring adequate levels of hydration throughout the fuel cell includes humidifying one or both of the reactants before they enter the fuel cell. For example, the water produced at the cathode can be used, with an appropriate humidification device, to reduce the likelihood of dehydration of the anode or the PFSA ionomer membrane. One example of such a humidification device is a WVT unit (also referred to as a membrane humidifier) that extracts the moisture from a humid fuel cell flowpath (also referred to as a flow channel) and places it into a feedpath used to convey a reactant low in humidity. This is generally accomplished by using a WVT membrane that is disposed between adjacent high humidity and low humidity fluids. The membrane allows water vapor to pass through it from the higher humidity fluid to the lower humidity fluid while inhibiting the undesirable direct passage of inlet gases from the low humidity fluid to the outlet containing high humidity fluid without having first passed through the fuel cell. In one form of construction, this membrane may be attached to a diffusion media layer (also called a gas diffusion media (GDM)) that is generally similar (with the exception of the catalyst layer) to the diffusion layer of the MEA discussed above. Such a membrane and diffusion media layer combination may be referred to as a separator, a separator plate, or a membrane humidifier assembly. Numerous such separator plates may be stacked together such that alternating layers facilitate the respective passages of the dry and humid fluids. In one form, a WVT unit is made up of alternating layers of wet side separator plates (also called wet side plates) and dry side separator plates (also called dry side plates).
It is known to manufacture a WVT separator assembly made up of a plastic plates with integral flow channels, where the gas diffusion and membrane layers are attached to the plastic plate using pressure sensitive adhesive (PSA). Such an approach is time consuming and costly to manufacture, repair or replace. Furthermore, use of a plastic plate increases the overall dimensions of the separator plate.
An exemplary membrane humidifier for a fuel cell system that does not involve a plastic plate is disclosed in U.S. Published Patent Application 2009/0092863 to Skala, which is owned by the Assignee of the present invention and is hereby incorporated by reference in its entirety. The device depicted in that application describes a membrane humidifier assembly for a WVT unit having top and bottom layers formed from a diffusion medium that is in turn formed from a glass fiber impregnated with an uncured resin. An array of substantially planar elongate ribbons is disposed between the top and bottom diffusion medium layers to provide reinforcement of, and maintain separation between, the top and bottom layers. While the Skala system resolves many of the shortcomings of previous WVT separator plate designs, it would be desirable to further reduce the overall size, weight and complexity of a WVT unit using a stack of separator plate assemblies.